1. Field of the Invention
This invention relates to a thermal image transfer recording medium which can yield images with high density and hardly causes decrease in the density even when it is used repeatedly, and to a production process thereof.
2. Discussion of Background
Recording apparatus, such as a printer and a facsimile apparatus, to which the thermal image transfer recording method is applied are now widespread. This is because the recording apparatus of this type are relatively small in size and can be produced inexpensively, and their maintenance is simple.
In the conventional thermal image transfer recording medium for use with the thermal image transfer recording apparatus, a single ink layer is merely formed on a support. When such a recording medium is used for printing images, those portions of the ink layer heated by a thermal head completely transfer to an image receiving sheet at only one-time printing. Therefore, the recording medium can be used only once, and can never be used repeatedly. The conventional recording medium is thus disadvantageous from the economical point of view.
In order to overcome the above drawback in the prior art, there have been proposed the following methods:
(1) A microporous ink layer is formed on a support so that a thermofusible ink impregnated in the ink layer can gradually ooze out as disclosed in Japanese Laid-Open Patent Applications 54-68253 and 55-105579;
(2) A porous layer is provided on an ink layer formed on a support so that the amount of an ink which oozes out from the ink layer can be controlled as disclosed in Japanese Laid-Open Patent Application 58-212993; and
(3) An adhesive layer is interposed between an ink layer and a support so that the ink layer can be gradually exfoliated from the support when images are printed as disclosed in Japanese Laid-Open Patent Applications 60-127191 and 60-127192.
However, the above three methods have shortcomings as described below.
When the above method (1) is employed, the ink cannot sufficiently ooze out after the repeated use of the recording medium. As a result, the density of printed images gradually decreases as the number of printing times increases.
Regarding the method (2), the mechanical strength of the porous layer is decreased when the size of the pore included therein is increased in order to increase the image density, and thus the ink layer is to peel off the support together with the porous layer.
As for the method [3), the amount of the ink layer which peels off the support cannot be controlled uniformly when images are printed.
Furthermore, most of the conventional methods have been developed for a serial thermal head for use in a recording apparatus such as a word processor. Therefore, when those methods are applied to a line thermal head for use in a recording apparatus such as a facsimile apparatus or a bar code printer, some troubles are brought about, for instance exfoliation of an ink layer, and decrease in image density when the recording medium is used repeatedly.
In addition to the above, in a thermofusible ink which is prepared by a conventional method and contained in an ink layer, its dispersed system is destroyed when a thermal energy is repeatedly applied to the ink layer by a thermal head. As a result, the optical density of the ink contained in the ink layer is decreased before the ink layer is transferred to an image receiving sheet. Therefore, the density of images transferred from such an ink layer is not sufficiently high for use in practice.
Under these circumstances, there has been greatly expected a thermal image transfer recording medium for use with a line thermal head, which can yield images with high image density and hardly causes decrease in the image density even when it is used repeatedly.